The present invention generally relates to machine tools, and more particularly relates to machine tools using lasers for cutting metal and other materials.
Laser-equipped machine tools are often used to cut parts from sheet metal and plate. In such machine tools a laser beam, concentrated by a focusing lens or mirror to a small diameter spot, is directed to position the focal point above, on or below the surface of the material to be cut. The laser beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece, with a pressurized gas being directed through the nozzle, typically coaxially with the laser beam, to assist making the cut. The pressurized gas interacts with the laser beam and material, facilitating the cutting process, and creates a high velocity stream that carries the melted material away from the cut.
Laser-equipped machine tools are usually Computer Numerically Controlled, and are manufactured in many configurations and sizes and with lasers of various types and power. The present invention has applicability to all such-types, but will be described in association with one configuration, xe2x80x9cflying opticsxe2x80x9d. In that configuration the cutting head is adapted for movement along one axis, such as the Y-axis which is mounted on a bridge adapted for movement in an orthogonal, X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce and cut the metal to form holes and shapes in the material, and then to cut the part from the material.
Many same or different parts of common thickness and material type may be cut from a sheet or plate. Such groups of parts are commonly called a nest. Left over material, after the parts have been removed, is called a remnant or a skeleton. A small piece of scrap that falls from a hole cut in a part is called a slug. Remains of material from the cut is called slag. Resolidified material clinging to the part is called dross. The mixture of slugs and slag residue from cutting sheet material is generally called scrap.
Parts can be categorized as two classes, those that have one or more holes or shapes cut within the part boundary and those that consist of a boundary only, no holes or shapes within the part. Nests are cut such that holes within a part are cut first, then the boundary is cut. This is to maintain control of the part until all internal features are cut to insure accuracy of the part. If the boundary were cut first the part could shift, or worse tip, on the table making it impossible to accurately cut features within the part without additional setups or intervention.
Sometimes parts have internal holes or features that could result in substantial scrap. It is common to reduce the amount of scrap by placing one or more smaller parts in these areas. It is possible to have several sub groups of parts that are within parts that are in turn within parts again, etc. until the most practical and efficient use of the material is made. The cutting process for these part groups is the same, in that the parts are cut from inside out. First the holes and internal features are cut in the smallest or furthest part toward the innermost part of the part group, then the boundary of that part(s) is cut, then the next innermost holes or internal features are cut, then the boundary of that part(s), etc., until the boundary of the top level part is cut. A nest could contain none, one, or several instances of groups with parts placed inside another to make efficient utilization of material.
Software programs exist for automatically creating nests of parts. Some automatically evaluate the percent of material utilization in the nest. If targeted utilization has not been achieved the program discards the nest, rearranges the parts and evaluates material utilization until a minimum percentage is achieved. After a satisfactory nest has been created the software will create the program for the laser cutting machine. Some such programming systems also have the ability to determine a cut path such that the cut or machine movement never has to cross the cut line. This is beneficial in reducing risk of the cutting nozzle colliding with parts that tip on the work support and project above the surface of the work being cut.
As higher power lasers with beam qualities suitable for cutting are developed, cutting machine technology advances to cut greater thicknesses of material. For example, it is now possible to cut steel plate 1 inch thick at 24 inches per minute with a cutting machine equipped with a 6-kilowatt laser.
When cutting thick carbon steel plate, the cut quality can rapidly deteriorate when a phenomenon characterized as xe2x80x9cself-burningxe2x80x9d occurs. Self-burning is a condition that occurs when control of the cutting process is lost and there is a thermal runaway producing a wide kerf which is still closed at the bottom.
The self-burning phenomenon will sometimes clear itself. Sometimes the clearing happens while cutting an edge or arc of the part, but it usually happens at a corner. Cutting velocity decreases when a corner is encountered. Often the program includes a dwell in a corner allowing the lagging tail of the cut to catch up with the leading edge of the cut.
Sometimes control of the cut can be regained by the machine operator intervening to reduce cutting speed with manual feedrate override. Often control of the cut cannot be regained without stopping the process.
When self-burning clears on its own or is cleared by the operator, it leaves a defect on the cut edge. When not noticed, self-burning can result in the production of unacceptable parts which must be reworked or scrapped.
The self-burning phenomenon is troublesome. The machine operator must watch the process and be ready to respond to problems or run the risk of cutting unacceptable parts. In this era of production the operator is often expected to run multiple machines unloading cut parts, loading fresh material, loading the next part program, or even unloading trucks and cleaning the area while the machines cut.
The inventor is aware of at least one method of dealing with self-burning on an automatic basis. It is based on the visible light generated at the cut when self-burning commences. The visible light is reflected back through the cutting lens into the beam path. During normal cutting little visible light is reflected. A sensor is placed above the cutting lens to sense the light reflected from self-burning. It sends a signal to the CNC. In response to the signal, the CNC stops the cut, repositions the cutting head a short distance back along the cut path, then restarts the cut and again attempts to cut through the problem area. CNC setup parameters determine the number of attempts made to make the cut. If the number of attempts is made without success, the CNC will stop the program, send an error message to the CNC operator""s screen, and wait for operator intervention.
While this method of dealing with self-burning is beneficial, it remains problematic. First, it detects self-burning after its onset, therefore, the best that can be expected is to produce a part with a small defect on the edge. Second, since self-burning has commenced, it reduces the chance of successfully cutting through the area seconds later.
The objective of the present invention is to provide a laser-equipped machine tool having ability to avoid self-burning.
Observation of the self-burning phenomenon and examination of parts and skeletons have led the inventor to postulate certain theories and to arrive at several conclusions, as follows.
When the sidewalls of the cut absorb too much heat from the process they melt and fall into the cut. The sudden rush of molten metal is too much to escape from the narrow kerf and closes the cut. The process then feeds itself. With the kerf closed all the heat produced by the laser beam remains in the kerf instead of being removed with material exiting the cut. Oxygen assist gas continues to feed the thermal runaway. This creates a boiling mass of molten steel some of which, in the form of sparks and globules of molten metal, is blown upward, out of the cut, by the assist gas. This makes the self-burning phenomenon visually very noticeable. There is also a noticeable change in the sound of the process.
The inventor has observed that the area in which self-burning occurs is often predictable. Self-burning usually occurs when the cut enters an area which has been recently heated by cutting a nearby feature, wherein the material remains hot. Experiments have shown that it is possible to monitor the temperature of the plate and thereby assess the risk of onset of self-burning when cutting into the area.
Tests were conducted in which an infrared thermometer was used to monitor the surface temperature of a plate while it was cut. The infrared thermometer was selected to assure it would not detect the laser beam. Monitoring the temperature directly in front of the cut path while cutting 12 mm thick construction grade carbon steel, it was observed that cut quality started deteriorating above 300xc2x0 F. Around 400xc2x0 F. the cut was lost due to onset of self-burning. In a test involving the same type material except 25 mm thick, it was observed that stability of the cutting process started deteriorating around 180xc2x0 F. Dross started forming on the lower edge above 200xc2x0 F. At 220xc2x0 F. there was high risk of onset of self-burning. After onset of self-burning the temperature rose, within two seconds, to 350-400xc2x0 F.
It was concluded that the temperature differences between 12 mm and 25 mm and the onset of cut deterioration and self-burning were due to the thickness of the part and the thermal conductivity of the material. It appears that the material was hotter toward the bottom of the part. Heat must therefore be transferred by conduction to the upper surface. The rate of conduction and thus the temperature measured at the surface is dependent upon the thickness and the thermal conductivity of the material.
Based on these observations and conclusions, the objective of this invention is to provide a method and apparatus to assess the risk of the onset of self-burning and to take corrective measures to avoid self-burning.
A more detailed object of the invention is to provide a method and apparatus to assess the risk of self-burning and to automatically take action such that onset of self-burning is avoided by aborting the cut, moving to another area of the plate to resume cutting, then after a time lapse sufficient to allow the aborted area to cool, returning to the abort point and resuming cutting the aborted program.
An object of the present invention is to provide a laser cutting tool with sensing devices capable of detecting a condition in the workpiece indicative of the risk of self-burning onset, and to control or abort the cut as the sensed conditions predict that self-burning is imminent.
It is an object of the present invention to provide a laser-equipped cutting machine having a control system which senses temperature conditions in the workpiece indicative of the risk of onset of self-burning, and a control system which aborts the cut in an ordered, recoverable manner, then repositions the cutting head to begin cutting in a different area of the workpiece.
It is a resulting object to produce a high power laser-equipped machine tool capable of producing high quality cuts in thick material with high efficiency.
A general but subsidiary object of the present invention is to use a non-contact temperature sensor to measure the plate temperature in the region of the cut to predict the risk of onset of self-burning.
A more particular object is to utilize one or more infrared temperature sensors, selected to assure the IR wavelength used will not detect the laser beam, to monitor the temperature of the plate while it is cut to assess the risk of onset of self-burning.
It is a feature of the invention that the cutting head is equipped with a temperature sensor that senses the temperature of the workpiece in the area of the cut. The control system has a cutting parameter library which includes material cutting parameters and specifically includes temperature limits for warning or aborting a cut to avoid the onset of self-burning. If the temperature of the workpiece approaches the limit temperature, steps are taken to terminate the cut. In order to enhance machine efficiency, if the cut is aborted, the control system repositions the cutting head to another feature, part or group of parts, and begins to cut at that point. Before repositioning the cutting head, sufficient information is captured that the cutting head can be returned to resume the cut.
It is a further feature of a preferred embodiment of the invention to provide a laser-equipped machine tool wherein the control system maintains two temperature limits, a warning temperature limit and an abort temperature limit. If the workpiece reaches the abort temperature limit, the cut is stopped immediately, and the CNC repositions the cutting head to a different feature, part or group of parts identified by program flags. The warning temperature limit is lower than the abort temperature limit. If the warning temperature limit is reached, the CNC allows the machine to finish cutting the feature, then sets a flag at that location, then moves the cutting head to another flagged location to resume cutting. If the abort temperature limit is reached while trying to complete the cut, the cut is immediately aborted, the CNC sets a flag at that location then moves the cutting head to another flagged location to resume.
It is another detailed feature that upon jumping to another flagged location or upon return to a previously aborted location, the CNC reads the plate temperature, compares the reading to stored warning and abort values to assure the temperature is within acceptable limits, and if it is not, moves the machine to another flagged location to proceed.
These and other objectives, and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.